1. Field of the Invention
This invention pertains to a method and device for generating a laser beam with uniform cross-section from a laser beam having either a Gaussian or a high order transverse mode intensity distribution.
2. Description of the Prior Art
There are many laser applications that required a uniform light intensity distribution on a unique focal plane. Some examples are photolithographic mask exposure, semiconductor laser annealing, laser etching, photographic scanning, laser illumination, data processing and many laser machining applications. However, the lasers' spatial intensity profile is typically either a TEM.sub.00 mode with Gaussian distribution or a high order transverse mode with no well-defined analytical distribution but a relatively uniform center and sloping edges.
Many techniques have been proposed in the literature to provide some forms of modification for these laser beam profile distributions. An aspheric mirror pair was patented to Kreuger in U.S. Pat. No. 3,476,463 to deviate a well-defined Gaussian profile to a more uniform intensity distribution. Two afocal doublet lenses were proposed by Shafer in "Gaussian to flat-top intensity distribution lens" Optics and Laser Technology, pg. 159, 1982, using a similar concept and method. The major shortcoming of the above two methods is their restriction to an incident Gaussian laser beam profile of fixed diameter. A more general method has been reported by M.R. Latta and J. Jain of IBM in "Beam-intensity uniformization by mirror folding" Optical Soc. of America 1983 Annual Meeting, Technical program pg. 43. This technique folds the incident laser beam onto the center portion of itself with a focusing lens and a wedge mirror tunnel. The shortcomings of this method are threefold. First, it requires a strong focus point on the very front end of the reflector tunnel in order to form the folding which is undesirable in high power applications; second, the interference effect between the folded components, particularly for a TEM.sub.00 mode or other highly coherent laser beam can produce large intensity variations; and third, the system is very sensitive to the wedge angles and positions of the wedge mirrors and an additional recollimating mirror is required.